1. Field of the Invention
The present invention relates to a polypropylene resin composition which can be readily coated, in particular an impact-resistant polypropylene resin composition suitably used in making exterior automotive trims such as a bumper and a method for coating a molded product of the polypropylene resin composition, in particular, an exterior automotive trim such as a bumper.
2. Description of the Related Art
Polypropylene resins have been widely used in various fields because of low specific gravity and high rigidity as well as their excellent heat resistance and resistance to chemical attack. However, it has been known that the polypropylene resins have insufficient capacity of being coated since they are non-polar by nature. For instance, when an automotive bumper of polypropylene is coated, the bumper must be degreased and washed with a halogen atom-containing organic solvent such as 1,1,1-trichloroethane, then coated with a primer layer as an underlying coating or irradiated with plasma and thereafter a top coat must be applied thereto in order to achieve a sufficient bond strength to the coating layer.
The foregoing coating techniques require many processes and are not economical from the viewpoint of treating time. In addition, it is feared that the halogen atom-containing organic solvents used for degreasing and washing the polypropylene resin articles may adversely affect the human body and environment and correspondingly it has been desired for the immediate development of a means for solving this problem. Moreover, it can be appreciated that the coating methods conventionally carried out are quite uneconomical while taking the time and expense devoted in the treatment of waste liquid into consideration.
Under such circumstances, there have been proposed, from the viewpoint of reduction in the number of coating processes and of labor savings, various techniques which make it possible to delete the process for applying an underlying coating or which are adapted for so-called primer-less coating in the coating of automotive bumpers of polypropylene. For instance, there have been known (1) methods in which a compound having a polar group is grafted on polypropylene (Japanese Patent Unexamined publication (hereinafter referred to as "J.P. KOKAI") Nos. Sho 62-64848 and Sho 62-119243); and (2) methods in which a polar substance is blended with polypropylene (J.P. KOKAI Nos. Sho 51-145553 and Sho 61-89239).
However, the methods (1) in which a compound having a polar group is grafted onto polypropylene suffer from such problems that the compounds used therein are toxic and that physical properties of the resulting resin composition are impaired. Moreover, the bond strength to a coating applied to the polypropylene articles is still insufficient. The methods (2) in which a polar substance is blended with polypropylene simply provide a polypropylene article having a bond strength which is insufficient for use as a material for making bumpers.
Further, both the methods (1) and (2) include a process for degreasing and washing polypropylene articles with a solvent (1,1,1-trichloroethane) as an indispensable process and, therefore, it is necessary to work out Measures to solve the problems of detrimental effects of solvents on the human body and environments and of the treatment of waste liquid.